Not sure it really makes a difference for softwater fish, thats the problem - even if you swap Na for K, it'll still be high TDS water. All you are really doing is changing whether your hardness test kit can read it or not.
EDIT: Err, apologies - changing which test kits can read it or not, K isn't read by GH kits.

It might be of benefit to the plants in the mbuna tank (if they are lacking K), but again, not sure it matters whether it is Na or K to the mbuna._________________Fishing in the Rivers of Light